In a publication entitled "Preparation and characterization of polymer dispersed liquid crystal films using photosensitive polymers", Die Angewandte Mackromolekulare Chemie 231 (1995) 109-121 (Nr. 4051), J.-H. Liu, M.-Z. Chen and Y.-F. Wang report on the preparation of polymer dispersed liquid crystal (PDLC) films using a photosensitive technique.
As is reported by these authors, a light scattering state in an a.c. electric field off state depends on optical heterogeneities such as a spatial distortion of nematic directions and/or mismatching in refractive indices of the compounds. The light scattering and light switching properties of the composite films are decisively influenced by the phase separation structure of the composite films. The size of a liquid crystal domain (channel) can be controlled on the basis of the solvent evaporation rate or the curing rate during the separation of the composite film.
The authors employed commercial liquid crystals ZLI-2444, 2452, 2459, and BDH-E7 without further treatment. For the polymer matrix the authors used hydrophilic monomers of 2-hydroxythyl methacrylate (HEMA) and acrylic acid (Aa), hydrophobic monomers of methyl methacrylate (MMA) and styrene (St) that were purified by distillation in vacuo under nitrogen. 2-Methoxy-2phenylacetophenome (benzoin methyl ether, BME) was used without purification. The hydrophilic monomers were found to be suitable for the PDLC system. UV radiation was employed to photopolymerize the mixture, and it was found that sufficient UV irradiation was needed to complete the phase separation of the liquid crystals in the polymer matrix.
It is also known in the art to include isotropic dyes in an epoxy resin and liquid crystal when making optical shutters. The dye concentration can be adjusted to maximize the per cent transmission through the film between the on and off states. High contrast, colored displays have been made using an isotropic dye containing PDLC films and complementary colored backgrounds. Reference in this regard can be made to a publication entitled "Polymer Dispersed Liquid Crystals Incorporating Isotropic Dyes", SPIE Proceedings, 1080 (1989), J. L. West et al., and to a publication entitled "Characterization of polymer dispersed liquid-crystal shutters by ultraviolet/visible and Infrared absorption spectroscopy", J. Appl. Phys. 70(7), Oct. 1, 1991, pgs. 3785-3790, J. L. West et al. In U.S. Pat. No. 5,448,582, issued Sep. 5, 1995, entitled "Optical Sources Having a Strongly Scattering Gain Medium Providing Laser-Like Action", the inventor disclosed a multi-phase gain medium including an emission phase (such as dye molecules) and a scattering phase (such as TiO.sub.2). A third, matrix phase may also be provided in some embodiments. Suitable materials for the matrix phase include solvents and polymers. The material is shown to provide a laser-like spectral linewidth collapse above a certain pump pulse energy. FIGS. 9a and 9b illustrate a display system embodiment having, in one embodiment, a liquid crystal display (LCD) array that is positioned adjacent to a surface of a pixel plane comprised of pixels or sub-pixels of the inventive gain medium. The LCD array is arranged so as to be selectively energized for passing the emission from the pixels through to an observer. The pixels operate so as to be substantially non-saturable and to output electromagnetic radiation within a narrow band of wavelengths.
It is well known in the art to use security articles, for example, security threads in paper to hinder a non-authorized production of the paper or to authenticate already manufactured paper and/or a document or currency printed on the paper. Reference in this regard can be had to the following U.S. patents: U.S. Pat. No. 5,486,022, "Security Threads Having At Least Two Security Detection Features and Security Papers Employing Same", by T. T. Crane; U.S. Pat. No. 4,534,398, "Security Paper", by T. T. Crane; and U.S. Pat. No. 4,437,935, "Method and Apparatus for Providing Security Features in Paper", by F. G. Crane, Jr.